Dr. C. A. Kofoid on Platydorina. 541 



Antennae brownish towards the apex, stout, and placed 

 close to the top of the clypeus. Head rather long and flat 

 and wider than the thorax ; the eyes are almost parallel ; the 

 ocelli in a curve (• . ') and separated from the eyes by about 

 the same distance they are from each other. Clypeus short, 

 its apex transverse. Mandibles rufous, black at the apex; 

 the base thickly covered with depressed silvery pubescence. 

 Pronotum large, but not quite so long as the head ; it i3 

 depressed at the base, transverse at the apex. The entire 

 thorax is thickly covered with depressed silvery pubescence ; 

 its apex has an oblique slope. Wings short, not reaching to 

 the apex of the second abdominal segment, yellowish hyaline, 

 the apex near the middle of the radial cellule inf uscated ; the 

 radius is roundly curved from base to apes ; the second and 

 third cubital cellules are equal in length above and beneath ; 

 the first transverse cubital nervure has an oblique slope 

 on the upper half; the first recurrent nervure is received 

 shortly behind the second, beyond the middle of the cellule ; 

 the transverse median nervure is received at a distance — by 

 about its own length — beyond the transverse basal; the anal 

 nervure in the hind wing is appendiculated. Legs densely 

 pruinose, the tibias bare, the tarsi bearing thick spines, the 

 claws bifid. Abdomen with the segments bordered with 

 silvery pubescence ; it is about as long as the head and thorax 

 united ; the second ventral segment has a transverse furrow. 



The presence of a transverse furrow on the second (or third 

 according to one method of counting, the first segment forming 

 part of the median segment) brings this species into Bingham's 

 section " B. Ferreola Group." It has certainly no near 

 relationship beyond the abdominal furrow to such species as 

 P. Cameronii, Bingh. (fenestrata, Bingh., nee Smith). The 

 present species has some resemblance to the Pompilus vivax 

 group, but is easily known from them by the abdominal 

 furrow, by the transverse basal nervure being widely remote 

 from the transverse median, and by the short wings. 



LXXI. — Plankton Studies. — III. On Platydorina, a new 

 Genus of the Family Volvocidaa from the Plankton of the 

 Illinois River. By C. A. Kofoid, Ph.D.* 



[Plate VII.] 



The family Volvocida? is well represented in the plankton of 

 freshwater ponds and streams. Indeed, with the possible 



* From the ' Bulletin of the Illinois State Laboratory of Natural His- 

 tory,' vol. v. pp. 419-440. From a separate impression communicated by 

 the Author. 



Ann. & Mag. N. Hist. Ser. 7. Vol. vi. 36 



